Field of the Invention
The invention relates to a wire electric discharge machining apparatus having an automatic wire threader. Particularly, the invention relates to a wire electric discharge machining apparatus having an automatic wire threader with an anneal device for straightening a wire electrode.
Description of Related Art
A numerical control wire electric discharge machining apparatus includes an automatic wire threader for automatically threading a wire electrode through a start hole pre-drilled in a workpiece, or a kerf formed in a workpiece by machining, so as to stretch the wire electrode between a pair of wire guides.
For example, Patent Document 1 and Patent Document 2 disclose an automatic wire threader for straightening a wire electrode pre-stretched at a particular tension, by removing a curl through annealing the wire electrode, such that the small diameter wire electrode may be threaded through the start hole easier when the wire electrode is guided to an entrance of the start hole using a liquid jet for restraining.
The automatic wire threader having a function for straightening the wire electrode through annealing, supplies a current adapted for annealing the wire electrode (“anneal current”) to heat the wire electrode while in a state where a tension being applied is smaller than a tension applied during machining, so as not to break the wire electrode during heating of the wire electrode. In the invention, the conditions during annealing including a set value of the anneal current and a set value of the tension are called the anneal conditions.
Right after supplying a particular anneal current to the wire electrode, a temperature of the wire electrode will increase rapidly. Later, the temperature of the wire electrode stabilizes in a particular temperature range due to a temperature difference between the temperature of the wire electrode and a temperature of the surrounding. As a result, the wire electrode is annealed when a particular amount of time has passed after the anneal current is supplied to the wire electrode.
At this time, if the tension applied to the wire electrode directly after the anneal current is supplied is too small, then the tension will become smaller relative to the expanding elongation rate of the wire electrode caused by the rapid increase in temperature. Due to this, the wire electrode will become slack momentarily, such that an adequate straightness cannot be achieved. Therefore, the wire electrode is annealed while in a state where a tension with a set tension value of around a few hundred g is applied, such that the tension does not become too small and is also within a range in which the wire electrode does not break.
For example, Patent Document 3 discloses an automatic wire threader for suppressing a rapid increase in temperature during an initial annealing period, by supplying a current adequately smaller than the particular anneal current within a particular time period directly after starting annealing such that elongation of the wire electrode is suppressed. According to the invention of Patent Document 3, the wire electrode may be annealed in a straight state without breaking.